Isotenulin

aDepartment of Chemistry, The University of Tennessee at Chattanooga, Chattanooga, TN 37403, USA*Correspondence e-mail: kyle-knight@utc.edu

(Received 10 July 2013;accepted 16 July 2013;online 20 July 2013)

Isotenulin, C17H22O5, is a sesquiterpene lactone isolated from sneezeweed Helenium amarum. It crystallizes with two independent mol­ecules in the asymmetric unit. In each mol­ecule, two five-membered rings (cyclo­pentenone and lactone) are trans-fused to the central seven-membered ring. The five-membered rings each adopt envelope conformations. The seven-membered ring adopts a twist-chair conformation. In the crystal, the molecules are linked by C—H⋯O interactions, which generate a three-dimensional network.

Isotenulin, C17H22O5, is a sesquiterpene lactone isolated from sneezeweed Helenium amarum, a medicinal plant native to the southeastern USA. The crystal structure contains two independent tenulin molecules. These two conformations are distinguished by the way in which the five membered rings twist relative to each other, as measured by the magnitude of the dihedral angles at the cycloheptane-cyclopentenone and cycloheptane-lactone ring junctures. In the more twisted conformation, the dihedral angle of the substituents on the cycloheptane ring that form the cyclopentenone is 21.5 (1)°, while that of the lactone is 30.2 (1)°. Those values for the less twisted conformer are 18.1 (1)° and 28.2 (1)° respectively. The absolute configuration of all other stereocenters is that established for other sesquiterpene lactones (Mazhar-Ul-Haque et al., 1974).

H atoms were positioned geometrically at bond distances of 0.98, 0.99, 1.00 and 0.95 Å for methyl, methylene, methine and vinyl, respectively, and constrained to ride on their parent atoms with Uiso(H)= 1.2–1.5 Ueq(C).

Geometry. All e.s.d.'s (except the e.s.d. in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell e.s.d.'s are taken into account individually in the estimation of e.s.d.'s in distances, angles and torsion angles; correlations between e.s.d.'s in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell e.s.d.'s is used for estimating e.s.d.'s involving l.s. planes.

Acknowledgements

Acknowledgements are made to the National Science Foundation MRI Program (CHE-0951711) and the Grote Chemistry Fund at the University of Tennessee at Chattanooga for their generous support of our work.

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